The technique used for film formation in general includes a dry process and a wet process. Examples of the dry process include those of performing the film formation in a vacuum container, such as sputtering, and examples of the wet process include those of performing the film formation by using a coating material or by using plating or the like of effecting electrolytic or electroless deposition. Among these, in particular, the technique of performing the film formation by using a coating material requires no special equipment such as vacuum device, is easily after-applied, can be used over a wide range from laboratory scale to industrial mass production under similar conditions and by virtue of these characteristic features, is being used for imparting a design property to an article by the use of an ink, paint or the like, or imparting functions such as photocatalysis and corrosion resistance.
In such a coating material, for example, a particle group H blended for imparting an objective function such as photocatalytic ability, a component I for binding the particle group H to assure the strength as a film, a solvent J or the like for preparing a coating material from these, and an additive K for enhancing the dispersion stability and coatability as a coating material are blended in many cases.
The binding component I is not necessary in the case of using a relatively high temperature to allow melt-bonding of the particle group H as described in JP-A-07-155598, but the technique of using a high temperature has a problem that when the temperature of allowing for melt-bonding of the particle group H is very high as in ceramics, the application is limited only to a substrate having a melting point high enough to endure such a high temperature. Moreover, in the case of applying the film for creating an added value in a product already processed into a certain desired state or applying the film to a large member such as building material on site, the film-forming condition of 300° C. or more is not practical.
In this way, it is sometimes difficult to obtain a film having a practical strength by using only a particle group H but not using a binding component I. Therefore, an organic or inorganic binding component I capable of imparting the strength at a lower temperature is generally used.
As the ratio of the binding component I to the particle group H becomes larger, the film may have a higher strength. However, with increased blending ratio of the binding component I, the originally intended photocatalytic ability of the particle group H is generally inhibited. Furthermore, the active site of the particle group H for expressing the photocatalytic function is a chemically singular point in many cases, and the binding component I in particular often preferentially adheres thereto. Accordingly, in terms of the purpose of assuring the film strength and causing the film to satisfactorily exert the objective function, there is a trade-off relationship between the amount of the particle group H and the amount of the binding component I. In addition, the binding component I is expensive in many cases as compared with the particle group H, and its use in a large amount is not preferred.
As described above, the film-forming method using a coating material is useful but suffers from a problem that in order to form a film having a sufficiently high strength, the concentration of a substance irrelevant to the expression of function, such as binder, must be elevated and in turn, the photocatalytic ability of the particle group H is inhibited.
An object of the present invention is to provide a coating material capable of forming a film having both a sufficiently high film strength and a photocatalytic function by an easy method.